In U.S. Pat. No. 2,999,121-Mead, there is disclosed an expendable prefabricated thermocouple unit which may be easily and rapidly connected to temperature indicating equipment for making immersion temperature measurements in molten metal. After immersion in the bath of molten metal and the taking of a temperature measurement, the unit is discarded and replaced by another similar precalibrated unit. In U.S. Pat. No. 3,298,874-Davies, there is disclosed an improved expendable immersion thermocouple unit having a body member forming a cup portion which is filled with refractory cement during the manufacturing process. The refractory cement hardens to form a rigid body having electrical and heat insulating properties and to physically support the thermocouple.
Expendable immersion thermocouple units of the type disclosed in the Davies patent have enjoyed wide commercial use. It has been found, however, that an expendable immersion thermocouple manufactured in accordance with the teachings of the Davies patent may, under some circumstances, exhibit erratic temperature measurements during use and show mechanical failure of the refractory cement when removed from the molten metal bath. Such characteristics are most often encountered when the expendable immersion thermocouple devices are used with automatic insertion equipment that places the thermocouple in a vertical position during immersion in the molten metal bath.
It is not known why such expendable immersion thermocouples exhibit these characteristics. It is believed that during immersion in the molten metal bath that the mass of refractory cement, under the influence of the temperature of the molten metal, liberates gases due at least in part to the residual moisture in the cement. The rapid and violently escaping gases cause the temperature of the thermocouple junction to cool by forcing the cooler metal near the surface of the thermocouple unit to contact the thermocouple protection tube near the thermocouple junction to produce an erratic temperature measurement which is not the true reading of the temperature of the molten metal. It is further believed that the violent liberation of gas from the refractory cement causes a spalling of the surface of the cement.
Many different approaches have been used to try to overcome the problem referred to above but without success. It has been discovered that if the surface of the refractory cement is caused to have a preponderance of a high temperature refractory material such as alumina, conditions that produce an erratic temperature measurement and physical damage to the prior art expendable immersion thermocouple do not produce the erratic temperature measurement or the physical damage to thermocouple unit.
While it is not understood completely why the case or surface zone of the high temperature refractory material at the exposed surface of the refractory cement produces the improved performance, it is believed that the high temperature refractory material produces a thermal barrier so that during the very short immersion time, i.e. 5-10 seconds, the mass of refractory cement does not become sufficiently heated to produce the quantity of liberated gas experienced with the prior art expendable immersion thermocouples. Additionally, during the time of immersion the surface zone or skin containing the high refractory material becomes soft and allows the reduced volume of liberated gases to escape in a less violent manner than is obtained with a harder surface. The less violent escape will result in a lower velocity to reduce the mixing of cooled molten metal with the hot metal in the vicinity of the hot junction of the thermocouple. Finally, the soft surface also tends to provide a mechanical holding action on the unfired mass of refractory cement to prevent the spalling and mechanical damage encountered with expendable immersion thermocouples of the prior art.
It is an object of this invention to provide an improved expendable immersion thermocouple that does not exhibit erratic readings or mechanical breakage of the refractory surface when inserted vertically in a bath of molten metal.